Add SequencedTaskRunnerHandle to get a SequencedTaskRunner for the current thread / sequence.

base/threading/sequenced_task_runner_handle.h

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef BASE_THREADING_SEQUENCED_TASK_RUNNER_HANDLE_H_
+#define BASE_THREADING_SEQUENCED_TASK_RUNNER_HANDLE_H_
+
+#include "base/compiler_specific.h"
+#include "base/memory/ref_counted.h"
+
+namespace base {
+
+class SequencedTaskRunner;
+
+class SequencedTaskRunnerHandle {
+ public:
+  // Returns a SequencedTaskRunner which guarantees that posted tasks will only
+  // run after the current task is finished and will satisfy a SequenceChecker.
+  // It will only return a non-null value if one of the following conditions is
+  // fulfilled:
+  // a) The current thread has a ThreadTaskRunnerHandle (which includes any
+  //    thread that has a MessageLoop associated with it), or
+  // b) The current thread is a worker thread belonging to a SequencedWorkerPool
+  //    *and* is currently running a sequenced task.
+  static scoped_refptr<SequencedTaskRunner> Get();

[Sami, 2015/10/23 15:47:33]

Could this follow the same IsSet/Get contract of ThreadTaskRunnerHandle and
crash if Get is called without an active task runner? I think it's better to
crash early than later when the null pointer is used.

[Bernhard Bauer, 2015/10/23 16:20:25]

Done.

+
+ private:
+  DISALLOW_IMPLICIT_CONSTRUCTORS(SequencedTaskRunnerHandle);
+};
+
+}  // namespace base
+
+#endif  // BASE_THREADING_SEQUENCED_TASK_RUNNER_HANDLE_H_